Films of plastic material (e.g. polyethylene) conventionally used for such packaging shrink at temperatures below 100.degree. C. so that the use of a hot-water bath is convenient for this purpose. This is normally done with the aid of a conveyor belt dipping below the bath surface at an immersion point and rising above the water level at an extraction point, the speed of the conveyor being so chosen that articles carried thereon remain submerged for the proper period. When the articles to be sealed in such heat-shrinkable bags are of low weight, however, the packages or at least some of them may float on the surface and may therefore move only slowly if at all to the exit point. This will result in a prolonged exposure of articles to the hot water which could be detrimental especially in the case of foodstuffs.